Custom Webmail with my theme

Hi,

I can change Webmail login with a "Customise theme".

But when I want to custom the Webmail portal, I don't find a "Customize theme", only default theme "red", "green","blue", "light blue". How can I customize the Webmail with my template ? 

How can I change the css of "red", "green","blue", "light blue" ?

Can I manage with with an ftp access ?

Or something else ?

Can you help me please ?

Regards

Phil_H